1. Field of the Invention
The present invention relates to a hydrodynamic bearing used in a spindle motor or the like, to a method for manufacturing the bearing, to a spindle motor, and to a method for manufacturing the motor.
2. Description of the Related Art
The spindle motors installed in disk drives such as hard disk drives (hereinafter referred to as HDDs) have in recent years been hydrodynamic bearing motors, which involve non-contact rotation and therefore afford reductions in noise and NRRO.
As to the structure of these hydrodynamic bearing motors, FIG. 9 shows a conventional hydrodynamic bearing (see Japanese Utility Model Publication No. 2,525,216, for example), which comprises a shaft 103, a tapered portion 106, a sleeve 112, and a tapered bearing shell 108. The units on the shaft 103 side are fixed by a screw cramp. In this example, the tapered portion 106 is fitted to the shaft 103 having a similar tapered portion 104 that combines a radial bearing with a thrust bearing, and these are fixed with a threaded cover 114. The tapered bearing shell 108 is fitted to the sleeve 112. The space between the tapered portion of the shaft 103 and the tapered bearing shell is filled with a lubricating fluid.
In manufacturing a hydrodynamic bearing such as this, the required strength is achieved and the size of the product is reduced by fastening the above-mentioned units together by press-fitting, using an adhesive, or the like. Specific examples of fastening methods that have been used include shrink fitting and using an adhesive agent. All of these methods, however, entail problems; for example, there is dimensional change in the units, or the adhesive works its way into the parts and lowers the ultimate performance of the bearing.
Accordingly, in more recent hydrodynamic bearings (see Japanese Unexamined Patent Publication No. 2002-070849, for example), a large-diameter unit (first flange unit) 6 is formed as a first sealing unit integrally with part of a shaft 7, this is inserted into a sleeve 8, and a second flange unit 9 is press-fitted as a second sealing unit while a specific thrust gap is maintained (see FIG. 2, for example). FIG. 2 is actually a diagram of the structure of the hydrodynamic bearing pertaining to the present invention, but is used here for the sake of describing the structure of a conventional hydrodynamic bearing. However, even with this structure, when the second flange unit 9 is press-fitted to the shaft 7, the press-fitting produces burrs, and these burrs can find their way into the bearing parts and diminish the performance of the bearing, although the incidence of this problem is lower here. It is because of this that welding is the most commonly used method, since it does not produce any burrs (see Japanese Unexamined Patent Publication No. 2002-369438, for example).
Also, HDDs need to be even thinner and more compact. To reduce the size and thickness of a HDD, the spindle motor that rotates the disk must be made smaller and thinner. FIG. 13 is a cross-sectional view of a spindle motor in a conventional example (see Japanese Patent No. 3,282,945).
In FIG. 13, a sleeve 33 is provided in the middle of a housing 31, and a shaft 34 is inserted rotatably in a bearing hole of the sleeve 33. The sleeve 33, the shaft 34, and a thrust plate 35 constitute a hydrodynamic bearing that is known in this field of technology, and the shaft 34 and the sleeve 33 rotate in a non-contact manner. A rotor hub 32 is attached to the shaft 34. A magnet 36 is attached to the inner periphery of the rotor hub 32, and a magnetic disk 39 is attached to the outer periphery. The shaft 34 has a threaded hole 43, and a clamp screw 42 for fixing a clamping unit 41 is threaded into the threaded hole 43. The clamping unit 41 serves to hold the magnetic disk 39 in place. The magnet 36 rotates the rotor hub 32 and the shaft 34 upon receiving drive force from a stator core 37 fixed to the housing 31.
With the above spindle motor, the rotor hub 32 must be tightly attached to the shaft 34. With a typical attachment method, the shaft 34 is press-fitted in the hole of the rotor hub 32. However, since the inside diameter of the hole of the rotor hub 32 and the outside diameter of the shaft 34 may include a certain amount of production error, press-fitting alone does not always provide a secure and tight attachment. In view of this, the shaft 34 is press-fitted into the hole of the rotor hub 32 and the two are welded together, which affords a tighter and more secure attachment. The method for attaching the rotor hub 32 and the shaft 34 will be described through reference to FIG. 14.
FIG. 14a is a detailed cross-sectional view of the main components, and shows a first method for attaching the rotor hub 32 and the shaft 34. A rounded portion 44 or an inside-chamfered portion 45 has been formed on the square edge of the top face of the shaft 34. The inside diameter of the hole in the rotor hub 32 is slightly smaller than the outside diameter of the shaft 34, and when the shaft 34 is press-fitted into the hole of the rotor hub 32, the two are fixed by this tight fit. Next, a V-shaped concave part 46 formed with the rounded portion 44 and the inside-chamfered portion 45 is irradiated with a laser beam, which melts the shaft 34 and the rotor hub 32 near the concave part 46 and laser welds the two.
FIG. 14b is a detailed cross-sectional view of the main components, and shows a second method for attaching the rotor hub 32 and the shaft 34. First, a concave part 47 is formed around the hole of the rotor hub 32 so that the top part 34a of the shaft 34 sticks out. When the rotor hub 32 is attached to the shaft 34, the shaft 34 is press-fitted in the hole of the rotor hub 32, after which the square edge 48 is irradiated with a laser beam to laser weld the units.
With both of the above methods, the rotor hub 32 is fixed to the shaft 34 by press-fitting and laser welding, so a high fixing strength is obtained.
Nevertheless, the following problems are still encountered with the above conventional structures and methods.
When units are welded together, the solidification of the welds is accompanied by a change in the shrinkage stress of the units, which is a problem in that the specified dimensions of the units cannot be ensured. In particular, when the shaft and the flange that constitute the thrust bearing portion are welded, as disclosed in Japanese Unexamined Patent Publication No. 2002-369438, the welding causes the flange to move in the axial direction, and the heat imparted during welding also makes the flange warp in the vertical direction. Consequently, a problem is that the gap in the axial direction that is required with a thrust bearing cannot be ensured.
Also, with the first attachment method, when the rotor hub 32 and the shaft 34 are welded with a laser beam near the concave part 46, microcracks about 1 to 2 μm long and 1 to 1.5 μm deep develop in the surface of the welds. These microcracks contain tiny metal particles that cannot be readily removed by ordinary cleaning treatments. These tiny metal particles fly out of the microcracks as a result of vibration and so forth during operation of the spindle motor, stick to the surface of the magnetic disk, and adversely affect the recording or reproduction of data by the magnetic head. Also, welding fumes generated during laser welding (composed of an oxide micropowder produced from high-temperature metal during welding) may enter the threaded hole 43 and may cause contamination in the motor when assembling the HDD.
With the second attachment method, the flange can be prevented from going into the threaded hole 43 if the depth L of the concave part 47 and the length L that the top part 34a of the shaft 34 sticks out are increased. However, because the thickness of a spindle motor is restricted, if the length L (see FIG. 14b) of the top part 34a of the shaft 34 is too long, a fastening power between the shaft 34 and the rotor hub 32 gets decreased, and enough resistance to shock between them cannot be obtained. To prevent the problem, it is preferable also to minimize the length L. If the length L is decreased, though, welding fumes will be more apt to find their way into the threaded hole 43. Since the generation of microcracks is unavoidable, the same problems are encountered as with the first method.